Main landing gear legs

Fuel tanks

Seat

FIA race seat.
Seat made to this standard have all same side mounting, so seats are all compatible.
Fuselage width is critical, so check carefully of this dimension (shoulder height width.
Fixed mount, crash proof.

Other possibilities. With same mount. But check dimensions, so that it will fit cockpit.
Some FIA seats have wide horns at shoulder level, which are not necessary in aircraft and are not compatible with the space available.